1. Field of the Invention
This invention relates generally to formation fluid sampling, and more specifically to an improved formation fluid sampling module, the purpose of which is to bring high quality formation fluid samples to the surface for analysis, in part, by eliminating the xe2x80x9cdead volumexe2x80x9d which exists between a sample chamber and the valves which seal the sample chamber in the sampling module.
2. Description of the Related Art
The desirability of taking downhole formation fluid samples for chemical and physical analysis has long been recognized by oil companies, and such sampling has been performed by the assignee of the present invention, Schlumberger, for many years. Samples of formation fluid, also known as reservoir fluid, are typically collected as early as possible in the life of a reservoir for analysis at the surface and, more particularly, in specialized laboratories. The information that such analysis provides is vital in the planning and development of hydrocarbon reservoirs, as well as in the assessment of a reservoir""s capacity and performance.
The process of wellbore sampling involves the lowering of a sampling tool, such as the MDT(trademark) formation testing tool, owned and provided by Schlumberger, into the wellbore to collect a sample or multiple samples of formation fluid by engagement between a probe member of the sampling tool and the wall of the wellbore. The sampling tool creates a pressure differential across such engagement to induce formation fluid flow into one or more sample chambers within the sampling tool. This and similar processes are described in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. Nos. 5,303,775; 5,377,755 (both assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton).
The desirability of housing at least one, and often a plurality, of such sample chambers, with associated valving and flow line connections, within xe2x80x9csample modulesxe2x80x9d is also known, and has been utilized to particular advantage in Schlumberger""s MDT tool. Schlumberger currently has several types of such sample modules and sample chambers, each of which provide certain advantages for certain conditions.
xe2x80x9cDead volumexe2x80x9d is a phrase used to indicate the volume that exits between the seal valve at the inlet to a sample cavity of a sample chamber and the sample cavity itself. In operation, this volume, along with the rest of the flow system in a sample chamber or chambers, is typically filled with a fluid, gas, or a vacuum (typically air below atmospheric pressure), although a vacuum is undesirable in many instances because it allows a large pressure drop when the seal valve is opened. Thus, many high quality samples are now taken using xe2x80x9clow shockxe2x80x9d techniques wherein the dead volume is almost always filled with a fluid, usually water. In any case, whatever is used to fill this dead volume is swept into and captured in the formation fluid sample when the sample is collected, thereby contaminating the sample.
The problem is illustrated in FIG. 1, which shows sample chamber 10 connected to flow line 9 via secondary line 11. Fluid flow from flow line 9 into secondary line 11 is controlled by manual shut-off valve 17 and surface-controllable seal valve 15. Manual shut-off valve 17 is typically opened at the surface prior to lowering the tool containing sample chamber 10 into a borehole (not shown in FIG. 1), and then shut at the surface to positively seal a collected fluid sample after the tool containing sample chamber 10 is withdrawn from the borehole. Thus, the admission of formation fluid from flow line 9 into sample chamber 10 is essentially controlled by opening and closing seal valve 16 via an electronic command delivered from the surface through an armored cable known as a xe2x80x9cwireline,xe2x80x9d as is well known in the art. The problem with such sample fluid collection is that dead volume fluid DV is collected in sample chamber 10 along with the formation fluid delivered through flow line 9, thereby contaminating the fluid sample. To date, there are no known sample chambers or modules that address this problem of contamination resulting from dead volume collection in a fluid sample.
The present invention is directed to a method and apparatus that may solve or at least reduce, some or all of the problems described above.
In one illustrated embodiment, the present invention is directed to a sample module for use in a tool adapted for insertion into a subsurface wellbore for obtaining fluid samples. The sample module comprises a sample chamber for receiving and storing pressurized fluid. A piston is slidably disposed in the sample chamber and defines a sample cavity and a buffer cavity, the cavities having variable volumes determined by movement of the piston. A first flowline provides for communicating fluid obtained from a subsurface formation through the sample module. A second flowline connects the first flowline to the sample cavity. A third flowline connects the first flowline to the buffer cavity of the sample chamber for communicating buffer fluid out of the buffer cavity. A first valve capable of moving between a closed position and an open position is disposed in the second flowline for communicating flow of fluid from the first flowline to the sample cavity. When the first valve is in the open position, the sample cavity and the buffer cavity are in fluid communication with the first flowline and therefore have approximately equivalent pressures.
The sample module can further comprise a second valve disposed in the first flowline between the second flowline and the third flowline, and the second flowline can be connected to the first flowline upstream of said second valve. The third flowline can be connected to the first flowline downstream of the second valve. There can also be a fourth flowline connected to the sample cavity of the sample chamber for communicating fluid out of the sample cavity. The fourth flowline can also be connected to the first flowline, whereby fluid preloaded in the sample cavity may be flushed out using formation fluid via the fourth flowline. In one particular embodiment, the fourth flowline is connected to the first flowline downstream of the second valve. A third valve can be disposed in the fourth flowline for controlling the flow of fluid through the fourth flowline. The sample module can be a wireline-conveyed formation testing tool. In exemplary embodiments of the invention the sample cavity and the buffer cavity have a pressure differential between them that is less than 50 psi. In other exemplary embodiments of the invention, the sample cavity and the buffer cavity have a pressure differential between them that is less than 25 psi and less than 5 psi.
An alternate embodiment comprises a sample module for obtaining fluid samples from a subsurface wellbore. The sample module comprising a sample chamber for receiving and storing pressurized fluid with a piston movably disposed in the chamber defining a sample cavity and a buffer cavity, the cavities having variable volumes determined by movement of the piston. A first flowline for communicating fluid obtained from a subsurface formation proceeds through the sample module along with a second flowline connecting the first flowline to the sample cavity. A third flowline is connects the first flowline to the buffer cavity of the sample chamber for communicating buffer fluid out of the buffer cavity. A first valve capable of moving between a closed position and an open position is disposed in the second flowline for communicating flow of fluid from the first flowline to the sample cavity. A second valve capable of moving between a closed position and an open position is disposed in the first flowline between the second flowline and the third flowline. When the first valve and the second valve are in the open position, the sample cavity and the buffer cavity are in fluid communication with the first flowline and therefore have approximately equivalent pressures. The sample cavity and the buffer cavity can have a pressure differential between them that is less than 50 psi, less than 25 psi or less than 5 psi.
In another embodiment, the invention is directed to an apparatus for obtaining fluid from a subsurface formation penetrated by a wellbore. The apparatus comprises a probe assembly for establishing fluid communication between the apparatus and the formation when the apparatus is positioned in the wellbore. A pump assembly is capable of drawing fluid from the formation into the apparatus via the probe assembly. A sample module is capable of collecting a sample of the formation fluid drawn from the formation by the pumping assembly. The sample module comprises a chamber for receiving and storing fluid and a piston slidably disposed in the chamber to define a sample cavity and a buffer cavity, the cavities having variable volumes determined by movement of the piston. A first flowline is in fluid communication with the pump assembly for communicating fluid obtained from the formation through the sample module. A second flowline connects the first flowline to the sample cavity and a first valve is disposed in the second flowline for controlling the flow of fluid from said first flowline to the sample cavity. When the first valve is in the open position, the sample cavity and the buffer cavity are in fluid communication with the first flowline and thereby have approximately equivalent pressures.
The apparatus can further comprise a second valve disposed in the first flowline between the second flowline and the third flowline. The second flowline can be connected to the first flowline upstream of the second valve, while the third flowline can be connected to the first flowline downstream of the second valve. A fourth flowline can be connected to the sample cavity of the sample chamber for communicating fluid into and out of the sample cavity. The fourth flowline can also be connected to the first flowline, whereby any fluid preloaded in the sample cavity can be flushed out using formation fluid via the fourth flowline. The fourth flowline can be connected to the first flowline downstream of the second valve and can comprise a third valve controlling the flow of fluid through the fourth flowline. The apparatus can be a wireline-conveyed formation testing tool.
The inventive apparatus is typically a wireline-conveyed formation testing tool, although the advantages of the present invention are also applicable to a logging-while-drilling (LWD) tool such as a formation tested carried in a drillstring. The pressure differential between the sample cavity and the buffer cavity can be less than 50 psi, less than 25 psi or less than 5 psi.
Yet another embodiment of the present invention can comprise a method for obtaining fluid from a subsurface formation penetrated by a wellbore. The method comprises positioning a formation testing apparatus within the wellbore, the testing apparatus comprising a sample chamber having a floating piston slidably positioned therein, so as to define a sample cavity and a buffer cavity. Fluid communication is established between the apparatus and the formation and movement of fluid from the formation through a first flowline in the apparatus is induced with a pump located downstream of the first flowline. Communication between the sample cavity and the first flowline, and between the buffer cavity and the first flowline are established whereby the sample cavity, buffer cavity and the first flowline have equivalent pressures. Buffer fluid is removed from the buffer cavity, thereby moving the piston within the sample chamber and delivering a sample of the formation fluid into the sample cavity of a sample chamber. The apparatus is then withdrawn from the wellbore to recover the collected sample.
The method can further comprise flushing out at least a portion of a fluid precharging the sample cavity by inducing movement of at least a portion of the formation fluid though the sample cavity and collecting a sample of the formation fluid within the sample cavity after the flushing step. The flushing step can be accomplished with flow lines leading into and out of the sample cavity. Each of the flow lines can be equipped with a seal valve for controlling fluid flow therethrough. The flushing step can include flushing the precharging fluid out to the borehole or into a primary flow line within the apparatus. The method can further comprise the step of maintaining the sample collected in the sample cavity in a single phase condition as the apparatus is withdrawn from the wellbore.
In one particular embodiment the formation fluid is drawn into the sample cavity by movement of the piston as the buffer fluid is withdrawn from the buffer cavity and the expelled buffer fluid is delivered to a primary flow line within the apparatus. The pressure differential between the sample cavity and the first flowline can be less than 50 psi, less than 25 psi, or less than 5 psi. The fluid movement from the formation into the apparatus can be induced by a probe assembly engaging the wall of the formation, and a pump assembly that is in fluid communication with the probe assembly, both assemblies being within the apparatus.